Stanford Research Systems RGA100, Operating Manual And Programming Reference

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3-14 Ion Detector
SRS Residual Gas Analyzer
attracted away from the FC and strike the cone at high velocity producing electrons by
secondary electron emission. The secondary electrons are subsequently accelerated down
the four channels and produce more secondary electrons. For each ion entering the cone
of the CDEM, and depending on the bias voltage applied, up to 10
7
electrons come out at
the back end and are picked up by a grounded plate (the CDEM anode). The resulting
electron current is proportional to the ion current and is measured by the same
logarithmic electrometer used in the FC measurements. The sign of the electron
multiplier currents is reversed before the current value is sent out over RS232 so that the
computer does not need to do any sign flipping on the currents received when the CDEM
is activated.
The gain of the electron multiplier in the RGA is a function of the bias voltage and is
measured relative to the FC signal. The following figure shows a typical
““gain
characteristic ”” (i.e. gain vs bias voltage) curve obtained for H
2
O
+
ions at 18 amu.
1
10
100
1000
1E4
1E5
1E6
1E7
1E8
Gain
1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200
HV Bias (V)
Figure 9 Typical CDEM Gain Characteristic Curve
Very high gain values can be obtained and controlled with voltages smaller than -2500
Volts.
The sensitivity of the spectrometer increases with the gain of the electron
multiplier . Higher sensitivities provide lower minimum detectable partial pressures and
faster spectral scans. Minimum detectable partial pressure limits as low as 5
.
10
-14
Torr are
commonly achieved.
However, the increase in sensitivity is obtained at the price of limited dynamic range,